1. Field of the Invention
This invention relates to a method for driving a vibrator having a resonance frequency with the resonance frequency intermittently, and particularly relates to a method for driving powder feeders and a powder feeder capable of performing such driving method.
2. Description of the Related Art
In the case that a vibrator having a resonance frequency is driven in the resonance region (resonance point) continuously, it is difficult to control accurately the output such as the amplitude at the resonance point by way of a driving force (for example driving voltage). In view of such difficulty, a method in which the output is controlled by supplying driving force intermittently has been used. For example, a driving voltage is applied intermittently to control the operation time per one cycle (duty ratio) namely the time average output.
Such control is performed, for example, using a circuit shown in the block diagram of FIG. 8. In detail, the output from a resonance frequency oscillation circuit which is oscillating with the resonance frequency and the output from the duty ratio control clock generation means for changing the duty ratio are supplied to an AND circuit (product) to generate the product, and the product output is amplified and applied to the vibrator.
As a motor capable of such controlling, for example, an ultrasonic motor using a piezoelectric transducer has been known. In a piezoelectric transducer, the mechanical deformation of a piezoelectric element caused by electric energy is used to generate mechanical vibration of a vibrator and the output of the ultrasonic motor is changed by changing the duty ratio of the driving voltage.
For example, an ultrasonic resonator which generates both axial vibration (longitudinal vibration) and bending vibration generates elliptic oscillation with the resonance frequency. A pipe is attached to the end of the resonator, and powder is fed in the pipe, then the powder is moved in the certain direction, this mechanism can therefore be used as a powder feeder. In this case, also an AC driving voltage of the resonance frequency is applied to the resonator intermittently to control the feeding rate of the powder.
In some cases, driving force having resonance frequency is applied intermittently in order to obtain pulse vibration. In the case of a fish detector for investigating topography of sea floor or fishes by transmitting ultrasonic wave into water and by detecting reflected echo, a driving voltage of a resonance frequency is applied intermittently into water to transmit ultrasonic wave into water. On the other hand, after the transmission of ultrasonic wave the vibration is stopped, and an echo is received from water, and thus the fish detector serves as a sensor for catching the information in water.
Similar examples include an ultrasonic wave sensor for detecting the existence of some objects in air by emitting ultrasonic wave into air and detecting reflected ultrasonic wave from an object and an ultrasonic range finder for measuring the distance by measuring reflection time of the ultrasonic wave.
However, when a vibrator is driven as described herein above intermittently, after the stopping of driving force, the vibrator still vibrates in free vibration for a while. For example, a driving voltage of a resonance frequency is applied and then the voltage supply is stopped, when, the vibrator continues vibration and the amplitude of the vibration decreases gradually. Therefore, if the duty ratio is large, that is, the period while the driving voltage is not applied is short, the next vibration starts before this vibration ceases completely as shown in FIG. 9. FIG. 9A shows a cycle in which an active period (period while a driving voltage is applied) shown with High, and an inactive period (period while a driving voltage is not applied) is shown with Low. FIG. 9B shows the change of a driving voltage, and FIG. 9C shows the vibration of a vibrator.
On the other hand, when a vibration starts with a resonance frequency, the amplitude of a vibrator increases gradually up to the stationary amplitude. Therefore as shown in FIG. 9C, it takes some time from the starting of driving with a resonance frequency to the saturation of amplitude (stationary amplitude) through the gradual increase of the amplitude. Accordingly, even if the vibrator is driven by the driving wave as shown in FIG. 9B, the magnitude of vibration of a vibrator is only changed as shown in FIG. 9C and the vibrator is not ceased. Further, after a starting of driving, the amplitude increases gradually, therefore the vibration wave form is greatly different from the driving wave form shown in FIG. 9B.
To the contrary as shown in FIG. 10A, in the case that the duty ratio is small, that is, the period while a driving voltage is applied is short, the driving period ends before the vibration reaches the sufficient amplitude (refer to FIG. 10C), and the vibrator continues free vibration for a while after ending of a driving period. FIGS. 10A, B, and C represents the same references corresponding to figures in FIG. 9 respectively.
Accordingly, the vibration wave form similar to that of the driving wave form can not be obtained in the intermittent vibration of a vibrator having the resonance frequency. Such vibration wave form does not provide the output which is proportional to the duty ratio, and the linearity of output of apparatuses such as ultrasonic motors and powder feeders which utilize such vibration is adversely affected.
Further, a resonator of a fish detector and ultrasonic range finder does not output the rectangular ultrasonic output pulse, and the gradually increasing wave form causes an error of the time difference between the transmission and receiving of the reflected wave, the long lasting free vibration prevents the measurement of the reflected wave.
The present invention has been accomplished in view of these problems, and it is an object of the present invention to provide a method for driving a vibrator in which, for driving intermittently a vibrator having a resonance frequency with the resonance frequency, the vibration of the resonance frequency is attenuated promptly after the ending of the driving with the resonance frequency and the vibration of the resonance frequency increases promptly after the starting of the driving with the resonance frequency.
Further, it is another object of the present invention to provide a method for driving a powder feeder using an ultrasonic motor which is easily controllable to feed powder and to provide a powder feeder.